Current carrying coils

1. Feb 11, 2008

buckeyes

Two coils have the same number of circular turns and carry the same current. Each rotates in a magnetic field in a setup similar to the square coil in the figure below. Coil 1 has a radius of 5.5 cm and rotates in a 0.17 T field. Coil 2 rotates in a 0.42 T field. Each coil experiences the same maximum torque. What is the radius (in cm) of coil 2?

http://www.webassign.net/CJ/21-21.gif

here is what i tried.
i found the area of the first coil by using a=(pi)r(squared) =(95.03)
next i used the formula t=NAIB for both coils. since i know they both have the same N, I, and t, i can combine the two formulas ending up with AB=ab (cap letters represent 1st coil, lowercase letters represent coil 2) i know all variables except A. now i can find A=38.46. now i can find r of the second coil using A=(pi)r(squared) i found r to be 10.99cm but this is wrong. where did i mess up?

2. Feb 11, 2008

blochwave

I believe everything looks good up to A=38.46

So the area's smaller than the other coil's area, but the radius is bigger? Check your math again

3. Feb 11, 2008

buckeyes

got it. thank you. :)